1. Field of the Invention
The present invention relates to semiconductor laser devices having compound semiconductor layers composed of III-V group nitride based semiconductors (hereinafter referred to as nitride based semiconductors) such as BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), TlN (thallium nitride) or InN (indium nitride) or their mixed crystal and methods of fabricating the same.
2. Description of the Background Art
As light sources of optical disk systems for recording and reproducing optical disks, semiconductor laser devices have been employed. Particularly, expectations have been growing that nitride based semiconductor laser devices such as GaN based semiconductor laser devices are used as light sources for high-density optical disk systems such as new generation digital video disks.
FIG. 6 is a schematic sectional view showing an example of a GaN based semiconductor laser device.
In a semiconductor laser device 110 shown in FIG. 6, an AlGaN buffer layer 32, an undoped GaN layer 33, an n-GaN layer 34, an n-AlGaN cladding layer 35, a multi quantum well light emitting layer (hereinafter referred to as an MQW light emitting layer) 36, a p-AlGaN cladding layer 37, a p-first GaN cap layer 38, a current blocking layer 39 composed of n-AlGaN and having an opening, and a p-second GaN cap layer 40 are stacked in this order on a sapphire substrate 31.
The semiconductor laser device 110 has a ridge waveguide structure. A ridge portion is constituted by the p-AlGaN cladding layer 37 and the p-first GaN cap layer 38. The opening of the current blocking layer 39 is formed on the ridge portion.
A partial region from the p-second GaN cap layer 40 to the n-GaN layer 34 is etched, so that an n type electrode 50 is formed on the exposed n-GaN layer 34. On the other hand, a p type electrode 51 is formed on the p-second GaN cap layer 40.
In the semiconductor laser device 110, a current injected from the p type electrode 51 is narrowed by the current blocking layer 39. Therefore, a striped region in the ridge portion under the opening of the current blocking layer 39 becomes a current injection region, as indicated by arrows in FIG. 6. Consequently, a region 41 at the center of the MQW light emitting layer 36 emits light. Further, the refractive index in the current blocking layer 39 composed of n-AlGaN is set to be lower than the refractive index in the p-AlGaN cladding layer 37 in the ridge portion, whereby the effective refractive index in a region 41 at the center of the MQW light emitting layer 36 is higher than the effective refractive index in a region on both sides thereof. Consequently, light is confined in the region 41 at the center of the MQW light emitting layer 36. Transverse mode control is thus carried out in the semiconductor laser device 110.
In the semiconductor laser device 110, low-noise characteristics are required at the time of reproducing the optical disk. In the semiconductor laser device 110 lasing in a single mode, however, laser light has strong coherence, so that noise occurs by light returned from the optical disk. Therefore, a semiconductor laser device in which a region having saturable light absorbing characteristics (hereinafter referred to as a saturable light absorbing region) is formed by forming a low current injection region in the MQW light emitting layer 36 has been proposed. In the semiconductor laser device, low-noise characteristics are achieved by subjecting the laser light to self-sustained pulsation.
FIGS. 7(a) and 7(b) are schematic sectional views showing an example of a semiconductor laser device having low-noise characteristics.
A semiconductor laser device 120 shown in FIG. 7(a) has the same structure as the semiconductor laser device 110 shown in FIG. 6 except for the following.
In the semiconductor laser device 120, when a ridge portion is formed by etching, steps are further formed in a p-AlGaN cladding layer 37 so that the width W3 of the upper step is smaller than the width W4 of the lower step. Consequently, a striped region having the width W3 in the ridge portion becomes a current injection region, and a saturable light absorbing region 42 is formed on both sides of the current injection region in an MQW light emitting layer 36. As a result, laser light is subjected to self-sustained pulsation.
On the other hand, a semiconductor laser device 130 shown in FIG. 7(b) has the same structure as the semiconductor laser device 110 shown in FIG. 6 except for the following.
In the semiconductor laser device 130, the etching depth in forming a ridge portion is controlled, to increase the thickness d of a p-AlGaN cladding layer 37. Consequently, a striped region in the ridge portion becomes a current injection region, as indicated by arrows in FIG. 7(b). By increasing the thickness d of the p-AlGaN cladding layer 37, the difference in the effective refractive index in the horizontal direction is decreased in an MQW light emitting layer 36. Accordingly, light oozes out in the horizontal direction into a region, excluding a region under the ridge portion, of the MQW light emitting layer 36. Consequently, a saturable light absorbing region 42 is formed on both sides of the current injection region in the MQW light emitting layer 36. As a result, laser light is subjected to self-sustained pulsation.
A nitride based semiconductor layer such as a GaN based semiconductor layer is chemically stable. Therefore, the nitride based semiconductor layer cannot be patterned by wet etching, unlike a GaAs based semiconductor layer used for the conventional semiconductor laser device emitting red light or infrared light, and must be patterned by dry etching such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion Beam Etching). In such dry etching, selective etching cannot be performed. Accordingly, it is difficult to control the etching with high precision. Consequently, it is difficult to accurately form the structures of the above-mentioned semiconductor laser device 120 and 130.